1. Field of the Invention
The present invention relates to a brake device which applies a braking force to a braking target such as a wheel. Priority is claimed on Japanese Patent Application No. 2011-229523, filed Oct. 19, 2011, the contents of which is incorporated herein by reference.
2. Description of Related Art
As a brake device that applies a braking force to a wheel, there is known a configuration that generates a braking force by pushing a brake shoe (damper) against the tread surface of a turning wheel. In this type of brake device, to prevent cumulative wear on the tread surface from widening the gap between the brake shoe and the tread surface, there is typically a configuration which incorporates an automatic gap adjustment mechanism (for example, Japanese Unexamined Patent Application, First Publication No. S 59-192666).
An automatic gap adjustment mechanism is one which, when widening of the gap increases the stroke of the brake shoe, the gap is kept constant by moving the initial stroke position (the position before braking) of the brake shoe towards the tread surface.
FIG. 9 shows an example of a brake device with an automatic gap adjustment mechanism. In this brake device 101, a push rod 128 which advances towards the braking side in the braking direction by means of a piston unit 108, pushes out a brake shoe 102 (see FIG. 11) connected to the leading end thereof.
The automatic gap adjustment mechanism includes a gap adjustment unit 140 and a cam 141. The gap adjustment unit 140 includes; a sleeve 127, a ratchet gear 144, an adjustment unit housing 145 serving as a casing, and an adjustment push rod 147. The sleeve 127 is axially rotatable about an axis extending in the braking direction X, and threadably engaged with the outer peripheral surface of the push rod 128 connected to the brake shoe 102. The ratchet gear 144 is attached to the outer periphery of the sleeve 127. A pawl 148 is provided integral with the adjustment push rod 147. The cam 141 has an incline surface which is inclined so that it approaches the push rod 128 in a direction towards the braking side X1 in the braking direction.
The adjustment unit housing 145 is movable together with the sleeve 127 in the braking direction X, and is configured to be prevented from axially rotating about the braking direction X by a guide (not shown in the figure).
When the brake device 101 is to generate a braking force, the sleeve 127 is advanced towards the braking side X1 in the braking direction by driving the piston unit 108, thereby pushing the push rod 128 threadably engaged with the sleeve 127 until the brake shoe 102 contacts the tread surface.
Next, the operation of a conventional automatic gap adjustment mechanism is described. FIG. 10A to FIG. 10D explains how the adjustment push rod 147 moves when the sleeve 127 advances towards the braking side X1 in the braking direction. In FIG. 10A to FIG. 10D, the depth direction on the page corresponds to the braking direction X.
First, with advancing movement of the sleeve 127 and the push rod 128, a roller 154 provided at the end of the adjustment push rod 147 rolls upon the incline surface of the cam 141. As a result, the adjustment push rod 147, from a pre-braking position shown in FIG. 10A, moves towards the ratchet gear 144 as shown in FIG. 10B. Then the pawl 148 disposed so as to engage the ratchet gear 144 moves with the adjustment push rod 147.
If the gap is less than a prescribed value, and the push rod 128 has advanced by less than the prescribed value, the amount of lift of the roller 154 (the movement of the adjustment push rod 147) is insufficient for the pawl 148 to ride over a tooth of the ratchet gear 144, and the ratchet gear 144 returns to the pre-braking state (FIG. 10A) without rotating.
Here, as shown in FIG. 10C, when the pawl 148 moves by a predetermined adjustment value or more (that is, wear of the tread surface causes the push rod 128 to advance by a prescribed value or more), the pawl 148 rides over a tooth of the ratchet gear 114, and engages with the tip of the next tooth. When braking of the brake device 101 is released from this state, then as the push rod 128 moves towards the opposite side X2 to the braking side in the braking direction, the adjustment push rod 147 also moves towards the opposite side X2 to the braking side in the braking direction, and by rolling over the incline surface of the cam 141, moves in a direction away from the ratchet gear 144 (downward on the page).
Therefore, as shown in FIG. 10D, the tip of the pawl 148 which moves together with the adjustment push rod 147 rotates the ratchet gear 144 by one tooth only. Due to the rotation of the ratchet gear 144, the sleeve 127 integral with the ratchet gear 144 via a key also rotates, and the push rod 128 threadably engaged with the sleeve 127 adjusts the position by moving towards the braking side X1 in the braking direction.
As shown in FIG. 11, the leading end section of the push rod 128 is attached to a hanger 105 connected by a pin to the leading end of an arm section 107 provided on the top of the casing 106 of the brake device 101. That is to say, because the hanger 105 swings about the leading end of the arm section 107, the stroke of the leading end section of the push rod 128 exhibits a curve whose arc is centered on the leading end of the arm section 107.
However, when repeated adjustments, occurring as wear of the tread surface (braking target) progresses, cause the initial stroke position of the push rod 128 to move towards the braking side X1 in the braking direction, a problem occurs in that the movement of the adjustment push rod 147 relative to the distance moved (stroke) by the push rod 128 and the gap adjustment unit 140 under braking is small. This phenomenon is described below.
First, the amount of lift of the adjustment push rod 147 when there is no wear of the tread surface is described.
FIG. 12A shows the adjustment push rod 147 at the initial stroke position in a case where there is no wear of the tread surface, and FIG. 12B shows the adjustment push rod 147 at the braking position. As shown in FIG. 12A and FIG. 12B, when the tread surface is unworn, and the initial stroke position of the push rod 128 is positioned sufficiently towards the opposite side X2 to the braking side in the braking direction, supposing that the incline surface of the cam 141 is inclined by 20 degrees with respect to the braking direction X, a 21 mm stroke of the push rod 128 causes the adjustment push rod 147 (roller 154) to lift by 7.9 mm. Specifically, a proportional relationship exists whereby the adjustment push rod 147 lifts by approximately 1 mm for every 2.6 mm of stroke of the push rod 128.
Moreover, for example, in a configuration where the pawl 148 rides over a tooth of the ratchet gear 144 when the adjustment push rod 147 lifts by 5.4 mm, gap adjustment takes place when the push rod 128 undergoes a stroke of 14 mm.
FIG. 13 shows the adjustment push rod 147 at the braking position in a case where wear of the tread surface has progressed (90 mm in this case). As shown in FIG. 13, when wear of the tread surface has progressed moving the initial stroke position 90 mm towards the braking side X1 in the braking direction, the curved stroke of the push rod 128 causes the central axis F of the adjustment push rod 147 to adopt a slight incline. In contrast, because the cam 141 is secured to the side of the casing 106 which is unaffected by swinging of the arm section 107, the stroke distance of the push rod 128 is no longer in proportion to the amount of lift of the adjustment push rod 147. That is to say, because the longitudinal direction of the push rod 128 is at an incline relative to the angle of inclination of the cam 141 and the expected lift direction, if the push rod 128 in the above example moves 21 mm in the braking direction X, the adjustment push rod 147 only lifts by 3.9 mm.
That is to say, in the brake device configured so a 5.4 mm lift of the adjustment push rod 147 causes the pawl 148 to ride over a tooth of the ratchet gear 144, the automatic gap adjustment mechanism does not work, resulting in an inability to maintain a constant gap between the tread surface and the brake shoe 2.